1. Field of the Invention
This invention relates to the formation of substantially homogeneous single crystal silicon for use in the production of silicon slices for manufacture of semiconductor devices and the like.
2. Description of the Prior Art and Background
In the manufacture of single crystal silicon or semiconductor grade, single crystals are normally pulled from a melt of polycrystalline silicon utilizing a seed crystal and, under standard and well known conditions, pulling a single crystal of silicon from the melt. The crystal pulling normally takes place in an inert atmosphere, such as argon, and at elevated temperatures, usually in the vicinity of 1410.degree. C. It is known that in the processing of silicon in the above described manner, the single crystal, from which slices are later taken, become contaminated with heavy metals such as iron, copper and the like which are found in the furnaces and other processing materials and equipment being utilized.
In order to getter these heavy metal impurities, it has been found that the addition of oxygen to the melt provides precipitated zones where the oxygen has been precipitated and which tends to getter the heavy metals. Furthermore, the addition of nitrogen to the melt appears to strengthen the crystal itself. It is therefore readily apparent that the controlled addition of dopants such as oxygen and nitrogen during single crystal growth of silicon is required in order to obtain a silicon slice that has improved physical strength and has internal defect and impurity gettering capabilities that are active during device manufacture. Furthermore, each slice provided from the formed single crystal of silicon must have exactly the same concentration of dopant and it must have excellent radial uniformity. This slice to slice control of dopant level can only be achieved during the crystal growth process. Current crystal growth techniques are not capable of introducing and controlling the required exact concentrations of nitrogen and oxygen throughout the crystal growth process.
The concentration of dopant in the crystal during the single crystal growth process is a direct function of the dopant concentration in the melt and the segregation coefficient of that impurity. If the segregation coefficient is greater than 1 (oxygen is 1.25), then the concentration of oxygen will be high at the top of the crystal and low at the bottom. The reverse is true for nitrogen in silicon crystal growth.
There is a thermodynamic equilibrium between the concentration of the dopant in the melt and the partial pressure of a chemically related gas species above the melt. If the partial pressure of this gas species can be precisely fixed during the growth process, then the melt concentration of that species will be fixed and the concentration of the associated dopant will thereby be fixed in the crystal.